The present invention relates to arc-furnace equipment and more specifically to means and methods to help an electrical arc to strike thus improving productivity, reducing operating cost and reducing flicker.
Industrial arc-furnaces are huge furnaces which are typically used to melt different metallurgical elements such as bulk iron coming from scrap. The bulk metal is melted by the intense heat radiating from a hot gas column produced between an electrode and the scrap by an electric arc. The arc-furnace is basically composed of a vat to retain the scrap and the melted metal; a set of electrodes to spark the arcs; a set of actuators to control the electrodes distance from the scrap; and a large current power supply (including a transformer equipped with a tap changer to select a voltage level) to supply the arc currents. When the melting is completed, impurities floating on the surface are skimmed or scraped from the surface and then, the liquid metal is retrieved from the vat for further processing.
Creation of an electric arc requires an ignition normally performed by making a contact between two electrodes: a cathode and an anode. The cathode then emits electrons that are accelerated towards the anode by an electric field applied between the electrodes. These electrons collide with the gas molecules within the gap to generate positively charged ions and negatively charged free electrons to form a conductive gas column between the electrodes allowing the current to flow. A gas conductive enough to allow a current to flow will be referred in this document as a plasma. As the current increases, more collisions are made and more ions and electrons are freed, thus increasing the conductivity and temperature of the plasma column. At the same time, the cathode is bombarded with more ions and heats up thus maintaining electron emission. The anode also heats up due to the impact of the incoming electrons. The emission, the bombardments and the series of collisions generate a voltage-drop that can be divided in three zones: the cathode voltage-drop; the anode voltage-drop; and the plasma column voltage-drop. An arc-furnace arc has a voltage-drop distributed, for the most part, along the plasma column. Therefore, the arc voltage-drop will mainly increase with the arc length, will diminish inversely to the plasma temperature, and will depend on the plasma gas composition.
When the furnace electric arc is interrupted, it leaves the plasma column in an initial ionized state whose lifetime is influenced by the rate of plasma temperature drop and composition. The ignition-voltage required to re-strike the electrical arc will increase with the degradation of the plasma state. If the plasma is lost, a dielectric breakdown or a temporary electrical contact will be required to recreate the plasma and restrike the arc.
The most commonly used arc-furnace is the three-phase AC current type. The furnace comprises an electrode for each phase, all disposed according to a triangular pattern in the vat. During operation, each electrode produces an arc having its other end in contact with the load of metal. All the electrodes of the AC arc-furnace are alternately anode and cathode. At each half cycle, the arc current must pass through a zero point in order to reverse. The intense heat radiating from each plasma column is proportional to the arc current and therefore will fluctuate in a synchronous manner. At the line frequency of 50 or 60 Hz and in a cold environment, there is not enough heat inertia to maintain the plasma temperature to preserve the ionized state. In this case, the plasma temperature will fluctuate according to the current flow and will affect its conductivity. This change in conductivity will then affect the voltage-drop as the current fluctuates. If we consider the state following a current peak while the arc burns in a cold environment, there will be a progressive increase of the voltage-drop at the electrodes end. This voltage-drop will rise up to the extinction-voltage value where the current reaches zero and the arc extinguishes. For the reverse arc current to ignite, the alternating voltage supply must then, in the reverse polarity, exceed the ignition-voltage, which is dependent on the plasma column ionized state (temperature) and on the anode and cathode condition. After re-ignition, as the arc current increases back, the gas column warms up again, and the voltage-drop progressively regains, in reverse polarity, a lower value equivalent to the voltage-drop of the precedent current peak. If we draw the evolution of the arc voltage, the ignition-voltage will be higher than the extinction-voltage because in between events, the plasma column has continued to cool down.
An AC arc at a frequency of 50 or 60 Hz and burning in a hot environment behaves differently. The plasma column remains hot therefore sufficiently ionized when the arc current reaches zero and extinguishes. The extinction/ignition-voltage level will be weakly affected and the evolution of the voltage-drop will show a shape between a sinusoidal and a square wave.
An AC arc-furnace works with a sinusoidal voltage power supply. In order to ignite the arc shortly after its extinction, the arc-furnace operates at a lower power factor making the voltage leading the current due to the leakage inductance in the supply path of the furnace. In many cases, a series inductance is even inserted on the primary side of the furnace transformer. Then, when the current reaches zero and the extinction occurs, there is an immediate application of the reverse polarity voltage from the supply source with the vanishing of the back emf in the inductance. If the supply voltage is higher than the ignition-voltage at this instant, the arc will strike immediately. If it is not the case, a delay will be introduced until the voltage supply catches up the ignition-voltage level. This delay introduces dead time periods in the arc current, which creates current-less time intervals. Even the amplitude of the current, as well as its RMS value is reduced in a way similar to a phase controlled dimmer. The impact on the power input of the furnace is impressive.
The behavior of the arc-furnace depends strongly on the environment in which the arc is burning. Normally, a melting process involves two phases. In the first phase, subsequent loads of scrap are poured in the vat for melting down. During that phase, the furnace operates mainly in a cold environment. The arcs are not stable as they move erratically and jump from one piece of scrap to another. Also, the continuous slipping and melting of the scrap affects the arc length and generates frequent short-circuits of the electrodes. The arc behavior continuously changes the plasma column length, which also introduces a continuous variation in the dead time period and the short circuits creates inrush currents in the furnace high current power supply. If the dead time period is prolonged, the ignition-voltage will eventually become too high for the furnace supply voltage to strike an arc and the plasma will be lost. When a complete extinction of an arc occurs, the electrode must then be moved towards the scrap to make a contact and reinitiate the arc. The touch of the contact generates a high inrush current until the electrode is moved away to have enough plasma length for the current to reduce. For the second phase of the process the arcs behave differently. The scrap is completely melted in a hot liquid bath and the arcs are burning in a hotter and more stable environment. Moreover, a foamy slag is used to improve arc stability. Contrary to the first phase, the arc length is more stable and easier to control even if the arc contains current-less time intervals.
In a DC arc-furnace, there is no change in the direction of the arc current so that only the dead time periods described above do not exist. However, in a similar way to the AC arc-furnace, the erratic movement of the arc in the first phase may stretch the arc length to a limit where the furnace voltage supply can no longer maintain the current in the plasma because the voltage-drop gets to high. When this occurs, the plasma current decreases, thus cooling the arc and reducing its conductivity thus causing the current decrease to run away until the arc is interrupted and the plasma is lost. Here too, the electrodes have to be lowered in order to make a contact with the scrap for striking a new arc with the accompanied inrush current. These arc interruptions are most likely to happen only in the first phase.
The operation of an arc-furnace causes supply current fluctuations in the utility line. The largest current fluctuations are produced in the first phase by both the AC and the DC arc-furnace. In an AC arc-furnace, the erratic movement of the arcs, the dead time periods, the inrush currents and the frequent extinction of the arcs create these current fluctuations. In the DC furnace, the inrush currents, the continuous change in the firing angle of the rectifier valves to compensate for the erratic movements of the arc as well as arc interruptions are the source of the supply current fluctuations. These fluctuations are the source of voltage fluctuations in the utility network. The utility company, to a certain extent, tolerates a part of this disturbance, known as flicker. The flicker is defined as the low frequency component of the voltage fluctuation encountered on the utility grid that cause disturbance to the eyes on such equipment as a light bulb. The amount of flicker is related to the ratio between the short circuit power of the supply network and the short circuit power of the arc-furnace. Unless this power ratio is sufficiently high, the furnace working point must be adapted during the process in order to constrain the flicker level within permissible limits. The flicker level can be reduced with the aid of static power compensators or by inserting a large inductor on the primary side of the arc-furnace transformer. Unfortunately, these apparatus are costly and the modifications to the arc-furnace supply are significant. Often, the arc-furnace operates with a low supply voltage and with the electrodes closer to the scrap. This will reduce the injected power until the scrap phase is completed and then, the power is increased as the burning arcs are more stable and the flicker is reduced.
An important aspect of an arc-furnace is its productivity. An arc-furnace is operated to produce the greatest number of heats possible. It is strongly related to the amount of power that can be transmitted to melt the scrap in a given time. The frequent complete extinction of the arcs, the dead time period encountered between each extinction and ignition, and the limited amount of flicker that can be tolerated, all contribute negatively to the arc-furnace production since these events all extend the melting process time.
Another important aspect of an arc-furnace is the production cost. For a fixed plasma current, the plasma voltage and heating capacity are proportional to the arc length. A longer arc will allow a plasma current reduction for a same amount of injected power. A smaller current has the advantage of reducing the electrode deterioration and consumption and also reduces the Joule losses in the supply circuit. Consequently, they will reduce the production cost.
These advantages will reduce the melting process time and the arc-furnace operating and maintenance cost and they will improve productivity.
A method to precipitate the striking of the electrical arc was disclosed in the international PCT application publication number WO 94/22279 (inventors Paulsson and Angquist). In this document, the apparatus improves the arc burning behavior by supplying to the electrodes a voltage pulse in connection with an interruption of the arc. After an immediate extinction of an arc, a voltage pulse is injected by discharging a capacitor or is induced in an inductor in the supply path by a temporary short-circuit to shorten the current-less intervals of the arcs. Unfortunately, for maximum efficiency, the apparatus requires the pulse to be injected at an optimum time delay following an interruption of the arc. The ignition may be unreliable because the striking of the main furnace arc may not happen or the main furnace arc current may not reach sufficient amplitude to maintain the arc after the voltage pulse has disappeared. (The main furnace arc is defined as the electric arc current supplied in the plasma column by the arc-furnace transformer). Moreover, during the delay preceding the injection of the pulse and during the time elapsing after the pulse disappears without the main furnace arc being struck, the plasma ionized state still continues to degrade. Also, the controller unit must track the arc-furnace output current to operate adequately. This method may prove to be reliable in the liquid bath phase of a heat but is difficult to apply in the first phase where the arc is erratic and most of the problems are encountered. The apparatus disclosed also requires a serial inductor at the output of the arc-furnace supply. Knowing that the supplied current is enormous, the inductor size is likely to be large. It is mentioned that the inductor could be avoided if the inductance consists of the inductance of the network, the furnace transformer and the connection lead. This option implies that part of the voltage pulse will propagate into the transformer and into the utility network, which is generally not desired nor allowed by the arc-furnace owner or the utility. If the inductor includes the furnace connection leads, then the power electronics must be located close to the electrodes where the environment conditions are extremely severe and where maintenance is problematic and must require a furnace shutdown.
An improvement of the arc-furnace can be accomplished by the application of a method to precipitate the striking of the electrical arc or avoid its interruption. This method offers the multiple following advantages:
The arc length or current can be increased;
the mean cyclic extinction period can be lowered;
the number of complete extinction event can be reduced;
the electrodes consumption can be reduced;
the Joule losses in the electrical circuit can be lowered;
the power factor can be increased;
and the flicker level can be lowered or the power can be increased.
It is an object of the present invention to provide a novel method and apparatus aimed to facilitate arc restriking in an arc-furnace, hence to obtain the advantages mentioned in the above background description without the drawbacks of the previous art.
It is a secondary object of the present invention to provide an apparatus to work in parallel with an arc-furnace without making major change to the arc-furnace structure and power supply.
It is another secondary object of the present invention to provide means to avoid excessive voltage amplitudes to be applied to the arc-furnace components.
It is another secondary object of the present invention to provide an apparatus in which the power electronics and the control unit are not exposed to the arc-furnace severe environment and can be accessed for maintenance without requiring an interruption of the furnace operation.
It is another secondary object of the present invention to provide an apparatus in which the controllability can be made simple and does not require an optimal time interval to act in order to be effective.
In accordance with a first aspect of the invention, there is provided an apparatus for improving re-striking in an arc-furnace having a large current conductor with a high-current power supply connected to one end of the conductor and an electrode connected to the other end of the conductor to produce an electrical arc for melting metal, the apparatus comprising a second quasi-continuous energy supply for maintaining a plasma link between the electrode end and the melting metal following an interruption of the electrical arc.
In accordance to another aspect of the invention, there is provided a method for melting metal in an arc-furnace using an electrical arc, comprising the steps of feeding a high current, from a high current power supply, using a large current conductor and an electrode, to the electrical arc, between the electrode and the melting metal of the arc-furnace and maintaining a plasma link between the electrode and the melting metal for a duration of an extinction of the electrical arc until a voltage of the high current power supply regains a value that will reestablish the electrical arc.